Devon Energy Corp. formally said it will buy Mitchell Energy & Development Corp. for about $3.14 billion in cash and stock, bolstering its reserves of natural gas and taking on a stable of gathering and processing assets in gas-rich north Texas.

Devon, an independent oil and gas producer based in Oklahoma City, said it will offer $31 in cash plus 0.585 Devon share for each share outstanding of Mitchell, based in The Woodlands, Texas. Devon will assume about $400 million in Mitchell debt.